1. Field of the Invention
The present invention relates generally to electro-optical inspection systems, and more particularly to a method or algorithm for automated photomask inspection to detect defects on optical masks, reticles, and the like.
2. Description of the Related Art
Integrated circuits are made by photolithographic processes which use photomasks or reticles and an associated light source to project a circuit image onto a silicon wafer. A high production yield is contingent on having defect free masks, reticles, and wafer surfaces.
Automated mask inspection systems have existed for several years. One of the earliest such systems used a laser that scanned the mask. Subsequent systems used a linear sensor to inspect an image projected by the mask using die-to-die inspection, i.e., inspection of two adjacent dice by comparing them to each other. Other systems have been developed that teach die-to-database inspection, i.e. inspection of the reticle by comparison to the database from which the reticle was made.
As the complexity of integrated circuits has increased, so has the demand on the inspection process. Both the need for resolving smaller defects and for inspecting larger areas have resulted in much greater speed requirements, in terms of number of pixel elements per second processed. The increased demands have given rise to improvements described in various publications and issued patents.
Photomasks are used in the semiconductor manufacturing industry for the purpose of transferring photolithographic patterns onto a substrate such as silicon, gallium arsenide, or the like during the manufacture of integrated circuits. The photomask is typically composed of a polished transparent substrate, such as a fused quartz plate, on which a thin patterned light blocking layer, consisting of figures, has been deposited on one surface. The patterned light blocking layer is typically chromium with a thickness of 800 to 1300 angstroms. This layer may have a light anti-reflection coating deposited on one or both surfaces of a patterned material, such as chromium, MoSi, or other material. In order to produce functioning integrated circuits at a high yield rate, the photomasks and the resultant semiconductor wafer surfaces must be free of defects. A defect is defined here as any unintended modification to the intended photolithographic pattern caused during the manufacture of the photomask or as a result of the use of the photomask. Defects can be due to a variety of circumstances, including but not limited to, a portion of the light blocking layer being absent from an area of the photolithographic pattern where it is intended to be present, a portion of the light blocking layer being present in an area of the photolithographic pattern where it is not intended to be, chemical stains or residues from the photomask manufacturing processes which cause an unintended localized modification of the light transmission property of the photomask, particulate contaminates such as dust, resist flakes, skin flakes, erosion of the photolithographic pattern due to electrostatic discharge, artifacts in the photomask substrate such as pits, scratches, and striations, and localized light transmission errors in the substrate or light blocking layer.
During the manufacture of photomasks, automated inspection of the photomask is performed in order to ensure freedom from the aforementioned defects. There are various methods for the inspection of patterned masks, reticles, or the wafer surface currently available. One of those inspection methods is a die-to-die comparison which uses transmitted light to compare two adjacent dies. These comparison-type inspection systems are quite expensive because they rely on pixel-by-pixel comparison of all the dies and, by necessity, rely on highly accurate methods of alignment between the two dies used at any one time for the comparison. Apart from their high costs, this method of inspection is also unable to detect particles on light blocking parts of the reticle which have the tendency to subsequently migrate to parts that are transparent and then cause a defect on the wafer.
Another method for inspecting patterned masks or wafers is restricted to locating particulate matter on the mask or wafer. It makes use of the fact that light scatters when it strikes a particle. Unfortunately, the edges of the pattern also cause scattering and for that reason these systems can in certain circumstances be unreliable for the detection of particles smaller than one micrometer.
Even with these newer photomask and wafer inspection techniques, it has discovered that certain aspects of the patterned wafer may present specific inspection challenges. For example, different wafer layers may include certain contacts, which are openings or holes in the layer enabling connection between transmissive elements on layers adjacent to the contact. In the case of contacts, small imprecisions in creation thereof may significantly harm the transmissive properties of the contact and should be avoided. The nature of contact creation is such that even small errors create large problems with transmissivity, and thus small errors in contact formation tend to have significantly larger adverse consequences than, for example, the presence of particles on the surface. A further problem with contact formation and errors associated with contacts is that or identifying contacts in the first place, as well as comparing a contact to known contacts. With respect to contact comparison, previous attempts to identify errors in contact formation used what was known as a “golden contact,” or ideal contact for comparison. The golden contact would have ideal properties and an inspected contact would be compared to the golden contact in a pixel-by-pixel comparison. In practice, however, the shape of the contact might be such that it had acceptable transmissive properties, but was somehow misshapen as a result of the fabrication process. Such a misinterpretation of the electrical properties of the contact would result in a good contact being classified as bad. Alternately, the pixel-by-pixel comparison depends on certain tolerance settings, and bad contacts could be flagged as good if the contacts fall within acceptable tolerance levels but ultimately fail to provide adequate transmissiveness characteristics. Further, contacts may intentionally have sizes and shapes which differ significantly from an ideal contact.
It would be beneficial to provide a system which did not include the drawbacks associated with previous contact inspection systems.